Manchester suicide bomber Salman Abedi was on the radar of British authorities as a Muslim extremist, but they failed to stop him before he massacred girls at a pop concert. It’s a recurring problem on both sides of the pond. US authorities also keep missing “known wolves” who were under suspicion before they attacked.

In fact, according to terrorism analyst Patrick Poole, at least 12 of the 14 Islamic terrorists who carried out attacks in the US during the Obama years had been previously investigated on extremism fears.

But authorities say don’t blame them. They say their hands are tied by official guidelines that restrict how long they can leave a case open without convincing a court there’s evidence of a crime.

If FBI agents can’t advance a case within six months, they’re obligated to close it, though they can reopen it if they obtain new information, such as suspicious changes in a suspect’s behavior, associations and travel.

Problem is, nothing’s formally ringing alarm bells within the FBI when a suspect does something that should trigger a reopening of a case, such as overseas travel to a jihadi hotspot — and this creates dangerous gaps in the monitoring of suspects.

“The DIOG (Domestic Investigations and Operations Guide) should be revisited to address this, allowing them to set trip wires to alert them to such changes,” former assistant FBI director Ron Hosko told The Post.

Omar MateenReuters

Rules also limit how aggressively agents can profile Muslim suspects and infiltrate their places of worship, where many of them are radicalized. Law-enforcement officials agree that having eyes and ears inside mosques is key to taking down jihadists before they go operational.

The NYPD had great success penetrating radical mosques before Muslim groups sued and Mayor de Blasio agreed to shut down such covert operations.

The FBI even joined one successful sting involving a Newburgh mosque. Intelligence found the mosque was creating an environment for radicalization, so the FBI planted an undercover informant inside. The operation led to the arrest of four Muslim members before they could act on plans to bomb synagogues and shoot down military planes.

Dzhokhar TsarnaevGetty Images

The key was being proactive. “If they had waited for the mosque clergy to report any suspicious behavior, the investigation would have failed and the terrorists would have been successful,” said Patrick Dunleavy, former deputy inspector general of the New York State prisons’ criminal intelligence division, who also worked with the NYPD’s intelligence division for several years.

The latest example is the cleric of the Manchester mosque Abedi attended. He maintains he argued with Abedi about his extremist views, including his support of ISIS. Yet he failed to report him to police or even kick him out of his mosque.

“There is a paucity of really good sources even among the respected Muslim community leadership,” retired FBI official I.C. Smith said.

Islamic centers are a necessary target of investigation, because too many of them act as recruiting stations for jihadists, even in America, asserts former CIA officer Clare Lopez, who heads research at the Washington-based Center for Security Policy.

“Mosques here are no different than mosques in Manchester or Molenbeek [Brussels],” she said. Many “are recruitment and training centers” and virtually all share “active hostilities against the US government and US law.”

The number of mosques in the US has nearly doubled over the past decade to more than 2,100, with the heaviest concentrations in Detroit, the New York-New Jersey area, Washington, DC, and Minneapolis, roughly in that order. Areas with large numbers of mosques tend to produce more terrorism suspects. It’s no coincidence that more people from the Detroit area are on the federal terror watch list than from any other American city except New York.

Hosko called such isolated Muslim enclaves “bubbling cauldrons that are more difficult for law enforcement to penetrate.”

“They need cooperation within mosques,” he added. Only, “too many of these killers are seen by family and friends ticking, and no one acts.”

Terrorists don’t go from thought to action overnight. It’s a process, one that’s steeped in Islamic doctrine. And there are identifiable steps and signposts along the way, but authorities aren’t trained to see them, mainly because Muslim-rights groups have convinced politicians to shut down that critical training.

Terrorists don’t go from thought to action overnight. It’s a process, one that’s steeped in Islamic doctrine. And there are identifiable steps and signposts along the way, but authorities aren’t trained to see them, mainly because Muslim-rights groups have convinced politicians to shut down that critical training.

“The problem is our FBI, Homeland Security and local law enforcement are not trained to recognize what a jihadi is, and what a jihadi looks like and sounds like and when to go on high alert because he’s about to go operational,” Lopez said.

Abedi displayed outward signs of radicalization. He is said to have become increasingly religious and interested in jihadist groups. Neighbors say he was chanting Islamic prayers weeks before the massacre. Also, friends say he stopped smoking pot and last year grew an Islamic beard and went to his mosque more frequently to pray — just like the Boston bombers, who also became more radical as they got more religious.

“Evidence exists to demonstrate that a greater level of adherence to Islamic law correlates to a greater likelihood of violence by that individual,” former FBI Agent John Guandolo said.

“If a Sharia-adherent Muslim is under investigation and then surveilled or seen at a strip club or bar, they may be planning to commit jihad in the immediate future and need to be seized immediately,” said Guandolo, whose Understanding the Threat LLC is the only government contractor in the country training local law enforcement in how to look for such red flags. He explains jihadists believe all sins are wiped away upon martyrdom.

He advises police to use simple charges — for traffic violations, fraud and domestic abuse — to obtain warrants for suspects who appear to be preparing for jihad. Unfortunately, manpower and budget constraints also hamper efforts to defuse human bombs in the Muslim community.

The FBI says it’s actively investigating more than 1,000 ISIS-related cases in all 50 states, plus more than 300 terrorism cases tied to refugees from Muslim countries.

“Let’s say you have 10 suspects in a small city that need to be watched. To watch them effectively, you have to monitor their social media and credit-card transactions in real time — 24/7/365 — and that requires three people working three shifts,” a US intelligence official told The Post. “The manpower demand adds up fast, and that’s expensive.”